bugfix in jaccard distance: distance not always correct when passing multiple strings (thanks to Robert Carlson)

Actually the last bug has not bitten anyone since it was masked by the second one (it was reported and fixed a long time ago but popped up again after fixing the second bug — hat tip to Hadley for testthat!). The second fix also ensures that stringdist’s memory allocator for q-gram storage is called fewer times which yields a speed gain in computation of q-gram based distances.